The present invention relates in general to an optical packet header, and more particularly to an apparatus for processing an optical packet header in an optical manner in an optical communication switching field.
Communication techniques may generally be classified into a transmission field and a switching field. Up to the present, the transmission field has made startling progress on the basis of the developments of a wavelength division multiple accessing (WDMA) technique and an electrical time division multiplexing (ETDM) technique to meet a rapid increase in demand for communication resulting from the influence of the Internet. With the developments of various optical techniques based on the advent of fiber optics, the transmission field has made another technical development that can advance an optical signal from an ingress node to an egress node with no optical/electric conversion.
The transmission field has been developed centering around an optical signal according to the spread of fiber-optic techniques as mentioned above, but the switching field is not so. In other words, the switching field has still employed such a conventional technique that converts an optical signal into an electrical signal to be switched and reconverts the switched electrical signal into the optical signal, resulting in a bottleneck degrading the overall communication rate.
Hence, an optical transparency must be secured in the switching field so that an electrical signal can be converted into an optical signal and the converted optical signal can be switched directly with no optical/electric conversion. On the other hand, the optical transparency can be secured on the basis of an optical packet switching technology, which has been studied in various ways for practical use. An optical packet header processing technique is one of several problems to be solved for the implementation of the optical packet switching technology.
However, although a variety of optical packet header processing techniques have been proposed up to now, they have not been put to practical use because they involve many problems in spite of their respective advantages.
Conventional optical switching techniques will hereinafter be described briefly.
FIG. 1 shows the construction of a conventional optical switch 10, which is operated in response to an electrical signal. Optical signals are inputted to respective input ports and then converted into electrical signals by respective optical/electric converters 11. Each of the electrical signals is stored in a packet unit into an electrical signal storage unit 12 in an appropriate manner. A head processor 13 decodes information in a header of each packet in an electric/electronic manner. A switch 14 analyzes a path of each packet using the header information and determines an output port of each packet in accordance with the analyzed result. Upon determining the output port, the switch 14 switches the corresponding packet to an output memory stack associated with the determined output port. Each output memory stack is implemented in a first in first out (FIFO) manner. As a result, each output memory stack outputs an earlier input signal, which is then converted into an optical signal by an associated electric/optical (E/O) converter 15.
FIG. 2 is a view illustrating the concept of a conventional optical packet switch 20. Each optical signal is inputted to an input port and then optically split into two optical signals by a beam splitter 21. The split optical signals are stored into an optical signal storage unit 22 and further transferred to a header processor 23, which decodes information in a header of each packet. Upon determining an output port of an optical packet as a result of the analysis, the header processor 23 operates an optical switch 24 to output the optical packet through the determined output port. At this time, the output optical packet is continuously maintained in an optical signal form through the entire construction of the optical packet switch 20 without being subjected to either optical/electric conversion or electric/optical conversion.
In such an optical packet switching field, the header processing is one of important technical elements and has been proposed in various manners. In FIG. 2, the header processor 23 is compelled to perform optical/electric conversion because the optical switch 24 processes an optical signal under an electrical control.
Such optical header processing techniques may greatly be classified into two methods, or the former performing optical/electric conversion and electrically processing the resultant signal and the latter optically processing a given signal and performing the optical/electric conversion with respect to the resultant signal. These methods have their respective merits and demerits, but such a common feature that they should store an optical packet in the form of an optical signal while processing its header. An optical path with a predetermined length, based on the uniformity in light velocity, is used for the storage of the optical signal, and the optical header must be processed rapidly within a given time.
An approach to the former method, or the optical header processing method which first performs the optical/electric conversion and then the electrical process, has been proposed by KEOPS [see: Guillemot, C., et al., xe2x80x9cTransparent Optical Packet Switching: The European ACTS KEOPS Project Approachxe2x80x9d, IEEE J. Lightwave Technology, vol. 16, No. 12, December 1998]. The overall length of an optical packet is 1646 nsec, which corresponds to 128 bytes at 622 Mbps. In the optical packet, a payload has a length of 1350 nsec and a header has a length of 14 bytes. The payload is subjected in rate to no particular restriction from several hundred Mbps up to 10 Gbps, but the header is fixed in rate to 155 Mbps. A synchronization pattern is appended to a head of the header for the processing of the header. A transmitted optical packet is optically radiated by a 1xc3x972 coupler and then subjected to optical/electric conversion. Subsequently, a clock is recovered from a header of the optical packet according to a synchronization pattern of the header and the contents of the header are decoded synchronously with the recovered clock. An address and other information can be written in the header as in a typical electrical method and thus be electrically restored. As a result, a sufficiently large amount of information can be secured, thereby enabling the general optical switch to be operated as shown in FIG. 2.
On the other hand, there have been proposed various methods of processing optical packet headers in an optical manner, as will hereinafter be mentioned. One method is a keyword method [see: Cotter, D., et al., xe2x80x9cSelf-routing of 100 Gbps packets using 6 bit xe2x80x98keywordxe2x80x99 address recognitionxe2x80x9d, IEEE Electronics Letters, vol. 31, No. 25, Dec. 7, 1995]. Each node in this keyword method is an add-drop node 30 as shown in FIG. 3. An n-bit header is created on the basis of n/2xe2x88x92n codes. A unique address is assigned to each node, which comprises a 2xc3x972 optical switch 34 for decoding a header of each input packet and determining whether to pass or drop each packet.
A header processor 33 acts to perform the header decoding operation, and an optical AND operation is used for the header decoding of the header processor 33. Namely, if one optical packet arrives at a specific node, then this node optically produces a complement address to a self address synchronized with a header of the optical packet. Thereafter, the specific node performs an optical AND operation for the optical packet header and the produced complement address, sequentially one bit by one bit.
At this time, provided that the header of the arrived optical packet has the same destination address as the self address of the specific node, the header address and the complement address of the node will have respective bit values opposite to each other. In this case, the optical AND operation results become 0 for all n bits contained in the header. As a result, the optical switch 34 in the node is crossed to drop the optical packet on the node.
However, unless the self address of the node and the address of the optical packet header are the same, the optical packet header and the complement address of the node will have at least one equal bit value. In this case, the optical AND operation result becomes 1. When at least one operation result is 1, the optical switch 34 is bar-shaped to pass the optical packet through the node.
FIG. 4 shows a method using an optical gate 45 instead of the optical switch 34 in FIG. 3. In a similar manner to that of FIG. 3, the method of FIG. 4 is adapted to perform the optical AND operation for the optical packet header and the node complement address and open the optical gate 45 only when the optical packet header and the node address are the same.
The above-mentioned keyword method is disadvantageous in that each add-drop node must produce a complement address to a self address whenever inputting an optical packet and perform an optical AND operation in accurate synchronization with a header of the input optical packet.
Another approach to processing the optical packet header in the optical manner is a method based on an optical code division multiple access (CDMA) technique, which is disclosed in U.S. Pat. No. 545,057, titled xe2x80x9cFiber-optic address detector in photonic packet switching device and method for fabricating the samexe2x80x9d, issued to Jong-dug Shin on Sep. 12, 1995, and shown in FIG. 5, herein.
In the above optical CDMA technique, as shown in FIG. 5, a 1xc3x97N beam splitter 51 splits one optical pulse into N optical pulses. These N optical pulses are transferred along paths with different lengths to an Nxc3x971 coupler 55, which then couples the optical pulses into one optical pulse. As a result, one optical pulse can be transformed into a set of optical pulses having predetermined time intervals. An optical fiber delay line matched filter 56 is used to produce a combination of n optical pulses from one optical pulse. At this time, each delay line has a length set to an integer multiple of a minimum time unit. That is, if the entire length of a header is N (N greater than n), then n optical pulses are arranged in the header in such a way as A=(Nxe2x88x921)!((nxe2x88x921)!/(Nxe2x88x92n)!).
Even in this method, a unique address is assigned to each node, and the optical fiber delay line matched filter is also used for the header decoding. Each address is determined according to the arrangement of delay line lengths. If an optical packet header is passed through the optical fiber delay line matched filter of each node, then the maximum (2Nxe2x88x921) optical pulses with different light intensities are newly produced on the basis of their rearrangement along delay lines. If one optical pulse forming an optical packet header has a light intensity of I after being twice passed through a 1xc3x97n beam splitter, an optical pulse with a light intensity of a maximum of nI can be produced when an address of a current node is equal to that of the optical packet header. As a result, recognizing the light intensity of an optical pulse makes it possible to determine whether the current node is a destination of the arrived optical packet. In a practically proposed structure, only one 1xc3x97n beam splitter can be used by coating the end of each optical delay line with metal to reflect an optical pulse.
The above optical CDMA method has an advantage in that a specific node need not produce a new optical signal once being assigned with a unique address, differently from the keyword method. However, the optical CDMA method is disadvantageous in that the number of produced addresses is very small as compared with the length of a header for securing orthogonality. Namely, the number of expressible addresses is smaller than (Nxe2x88x921)/{n(nxe2x88x921)} in an orthogonal optical code (OOC) where the intensity of light outputted as a result of the header processing for address retrieval is nI when two addresses to be compared are equal and I in other cases.
As mentioned above, the optical packet switch must process the optical packet header as rapidly as possible within a given time. For this reason, the optical process is preferable to the electric process. However, optical processing methods proposed up to the present are very complex in construction (like the keyword method) or significantly limited in the number of expressible addresses (like the optical CDMA method).
Therefore, the present invention has been made in view of the above problems, and it is an object of the present invention to provide an optical packet header processing apparatus for an optical packet switch which is simple in construction and can express a large number of addresses.
It is another object of the present invention to provide an optical packet header processing apparatus for an optical packet switch which is more advantageous in long-distance transmission than a conventional method based on a wavelength or light intensity in that it produces and processes a header in a time domain and which is simpler than a conventional keyword method and can accommodate a larger number of addresses in a header of the same length than those in a conventional optical CDMA method.
It is yet another object of the present invention to provide an optical packet header processing apparatus for an optical packet switch which is capable of generating the amount of information of one to several bits.
In accordance with one aspect of the present invention, the above and other objects can be accomplished by a provision of an optical packet header processing apparatus for processing a header of an optical packet expressing an address of a destination node to control a switching operation of an optical packet switch, comprising a first beam splitter for splitting the optical packet header into a predetermined number of optical packet header elements; a plurality of time interval detectors, each of the time interval detectors receiving a corresponding one of the optical packet header elements from the first beam splitter and outputting a detect optical pulse if a pair of optical pulses having a predetermined time interval therebetween are present in the received optical packet header element; and a plurality of optical pulse detectors for converting the detect optical pulses from the time interval detectors into electrical signals and transferring the converted electrical signals to the optical packet switch, respectively; whereby the optical packet switch determines the destination node in response to the electrical signals from the optical pulse detectors and outputs the optical packet to an output port corresponding to the determined destination node.
Preferably, the optical packet header has a total length of N bits and includes n( less than N) optical pulses, the optical packet header expressing the destination node as a combination of nxe2x88x921 optical pulses, and the first beam splitter is adapted to split the optical packet header into at least Nxe2x88x921 optical packet header elements.
Preferably, each of the time interval detectors includes a second beam splitter for splitting the corresponding optical packet header element from the first beam splitter into the pair of optical pulses; a direct line for passing one optical pulse of the optical pulse pair with no delay; a time delay line for delaying the other optical pulse of the optical pulse pair for a predetermined period of time; and an optical AND gate for outputting the detect optical pulse upon receiving the optical pulses from the direct line and time delay line at the same time.
In accordance with another aspect of the present invention, there is provided an optical packet header processing apparatus for processing a header of an optical packet expressing an address of a destination node to control a switching operation of an optical packet switch, comprising a first beam splitter for splitting the optical packet header into a predetermined number of optical packet header elements; a plurality of time interval detectors, each of the time interval detectors receiving a corresponding one of the optical packet header elements from the first beam splitter and outputting a first detect optical pulse if a pair of optical pulses having a predetermined time interval therebetween are present in the received optical packet header element; a first optical AND gate for performing an AND operation for the first detect optical pulses from the time interval detectors and outputting at least one second detect optical pulse as a result of the AND operation; and an optical pulse detector for converting the second detect optical pulse from the first optical AND gate into an electrical signal and transferring the converted electrical signal to the optical packet switch; whereby the optical packet switch drops the optical packet in response to the presence of the electrical signal from the optical pulse detector and passes it in response to the presence of no electrical signal from the optical pulse detector.
Preferably, the optical packet header has a total length of N bits and includes n( less than N) optical pulses, the optical packet header expressing the destination node as a combination of nxe2x88x921 optical pulses, and the first beam splitter is adapted to split the optical packet header into at least Nxe2x88x921 optical packet header elements.
Preferably, each of the time interval detectors includes a second beam splitter for splitting the corresponding optical packet header element from the first beam splitter into the pair of optical pulses; a direct line for passing one optical pulse of the optical pulse pair with no delay; a time delay line for delaying the other optical pulse of the optical pulse pair for a predetermined period of time; and a second optical AND gate for outputting the first detect optical pulse upon receiving the optical pulses from the direct line and time delay line at the same time.
In accordance with yet another aspect of the present invention, there is provided an optical packet header processing apparatus for processing a header of an optical packet expressing an address of a destination node to control a switching operation of an optical packet switch, comprising a beam splitter for splitting the optical packet header into a predetermined number of optical packet header elements; a plurality of time interval discriminators, each of the time interval discriminators receiving a corresponding one of the optical packet header elements from the beam splitter and outputting a discrimination signal if a pair of optical pulses having a predetermined time interval therebetween are present in the received optical packet header element; and a controller for determining the destination node in response to the discrimination signals from the time interval discriminators and transferring the optical packet to the determined destination node.